1. Field of the Invention
This invention relates to a circuit breaker in which the handle is blocked from movement to the off position when the contacts are welded closed.
2. Background Information
A common type of circuit breaker has a fixed electrical contact, and a movable electrical contact mounted on a movable contact arm. The contacts are closed and opened by rotating a handle between an on and off position, respectively. A latchable cradle connected to the movable contact arm by a spring operated toggle device is held in a latched position by a trip device. In response to predetermined overload conditions in the circuit breaker, the trip mechanism unlatches the latchable cradle and the spring operated toggle device rotates the movable contact arm to open the contacts. When the circuit breaker is tripped in this manner, the spring operated toggle device also moves the handle to a trip position intermediate the on and off positions. Thus, the handle provides a visual indication of the state of the circuit breaker. The circuit breaker is reset by moving the handle slightly past the off position to relatch the latchable cradle, and then to the on position to reclose the contacts.
It is possible under some overload conditions for the contacts of a circuit breaker to become welded closed. Under these circumstances, the trip device of the circuit breaker described above can respond to the overload condition by unlatching the latchable cradle. If the weld is of sufficient strength, the contact arm cannot be rotated and the contacts remain closed. However, it is possible to rotate the handle to the off position to relatch the cradle.
Several remedies for this condition have been proposed. In U.S. Pat. No. 3,525,959, the cradle is latched by a latch member which is engaged by the trip mechanism. To reset a tripped circuit breaker of this type, the knee pivot of the toggle which forms part of a spring loaded operating mechanism which trips the breaker, engages the latch member as the handle is moved to the off position to relatch the cradle. When the breaker is tripped, but the contacts are welded closed, the toggle remains erected and does not engage the latch member to relatch the cradle when the handle is moved to the off position. Thus, the cradle cannot be reset and the springs of the actuating mechanism maintain the handle in the on position when released indicating the true condition of the contacts.
In one embodiment of the circuit breaker of U.S. Pat. No. 3,614,685, a blocking member on the movable contact arm structure prevents the cradle from moving to a position at which it can be relatched by the trip mechanism when the handle is moved to the off position following a trip with the contacts welded closed. In another embodiment of this circuit breaker, a latch on the cradle engages a stationary part to prevent rotation of the cradle to the relatched position following a trip with the contacts welded closed. In both embodiments, the springs bias the handle to the on position under these conditions to indicate the real position of the welded contacts.
In U.S. Pat. No. 4,630,019, a handle yoke latch prevents rotation of the handle to the reset position if the contacts are welded closed. When the contacts are not welded closed, the movable contact arm structure rotates the handle yoke latch out of the path of the handle yoke to permit a tripped circuit breaker to be reset.
While the circuit breakers in U.S. Pat. Nos. 3,525,959 and 3,614,685 prevent relatching of the cradle following a trip with the contacts welded closed, and bias the handle to the on position to show that the contacts remain closed, the handle can still be moved to the off position. In some applications, circuit breakers are operated remotely by a motor operator. If the handle can be moved to the off position even though biased to the on position, the motor operator could hold the handle in the off position providing an indication that the contacts of the circuit breaker were open when in fact they were welded closed. In addition, in some installations, the circuit breaker is mounted in a cabinet with the handle mechanism extending through the cabinet door wall for external operation. It is possible for such a handle mechanism to have sufficient friction that the handle could remain in the off position despite the spring bias in the circuit breaker to the open position when the contacts were welded closed. In some installations, a hasp lock is provided to lock the circuit breaker in the off position. If the handle can be moved to the off position with the contacts welded closed, it is possible for the handle to be locked in the off position when in fact the contacts are welded closed. Obviously, this is not a satisfactory condition.
U.S. Pat. No. 3,849,747 discloses a miniature circuit breaker with a latchable cradle which is connected by a spring to a movable contact arm which in turn is connected to a handle. Since the handle is connected directly to the contact arm, it cannot be moved to the off position if the contacts are welded closed, and correspondingly, the handle cannot be relatched.
Commonly owned U.S. patent application Ser. No. 07/511,700, entitled "Circuit Breaker with Positive Indication of Welded Contacts" and filed on Apr. 20, 1990 discloses a circuit breaker in which an interference device in the form of a pin on a cross-bar joining the movable contact arm associated with each phase of the circuit breaker engages the operating member incorporating the handle to prevent rotation of the handle to the off position when the contacts are welded closed.
It is the primary object of the present invention to provide a circuit breaker with an improved arrangement for preventing movement of the operating handle to the off position when the contacts are welded closed.